1. Field of the Invention
The present invention relates, in general, to a device for protecting a bridge, and, more particularly, to an girder bridge protection device using a sacrifice member which functions to support loads normally applied to a bridge and to dissipate energy through plastic behavior caused by sacrificing a symmetrically structured main support member when a seismic load is applied, thereby protecting the remaining main parts of the bridge.
2. Description of the Prior Art
In the present specification, a sacrifice member is a member implementing the concept of a passive energy dissipation device. The member serves as a secondary element which plays a structural role while an earthquake does not occur. When a seismic load is applied, the member plays a role of passively dissipating energy generated in a structure and thereby improving girder performance.
Conventional techniques associated with a passive energy dissipation device or an girder bridge protection device have been disclosed in Korean Utility Model Registration No. 217048 (dated Jan. 5, 2001) entitled “Apparatus for preventing separation of super structure from continuous steel box bridge” and Korean Utility Model Registration No. 335443 (dated Nov. 28, 2003) entitled “Apparatus for supporting bridge”.
In the conventional art, various structures have been applied as the passive energy dissipation device. Examples of the typical devices developed so far include metallic yield dampers, friction dampers, viscoelastic dampers, viscous fluid dampers, tuned mass dampers, tuned liquid dampers, etc. (Soong et al., 2002).
The metallic yield dampers function to dissipate energy generated in a structure by a seismic load, using a nonlinear behavior characteristic of a metal. Devices which are generally used adopt an ADAS (added damping and stiffness) method in which an X-shaped or triangular steel plate is used so as to evenly distribute plastic deformation over the entire member. Other devices have a honeycomb-shaped configuration which is mainly adopted in Japan, employ shear panels, and are formed of lead, shape-memory alloy, etc. which are different from steel (Aiken et al., 1992).
Recently, in another type of metallic yield damper, an unbonded brace (tension/compression yielding brace) is used. The unbonded brace is composed of a steel section for dissipating energy by axial force and a tube filled with concrete to resist buckling due to compressive force (Wada, 1999; Clark, 1999; and Kalyanaraman et al., 1998).
The friction dampers serve as devices which dissipate energy generated in structures by seismic loads using frictional force generated between two objects. That is to say, the friction dampers dissipate energy using frictional force generated in the device by compressive and tensile force.
A hysteresis loop of the friction damper reaches a square due to the characteristic of coulomb friction. Using this hysteresis model, it is possible to analyze the behavior of the structure due to a seismic load (Pall et al., 1982; Gringorian et al., 1993; and Pall et al., 1993).
The viscoelastic dampers function to dissipate energy generated in a structure mainly using shearing deformation of copolymer, a glass material, etc. (Chang et al., 1994; Shen et al., 1995; and Lai et al., 1995).
The viscous fluid devices are largely divided into viscous walls and VF dampers. The viscous walls are devices in which energy is dissipated while a plate is moved between thin steel plates filled with viscous liquid. The viscous walls have been used for military and aviation purposes and recently have been applied to civil-engineered structures.
The VF dampers comprise a piston which is defined with an orifice and which moves in a cylinder filled with highly viscous material such as silicon and oil (Constantinou et al., 1993). The VF dampers function to dissipate energy generated by a seismic load, through the movement of the piston which is caused due to the operating principle of the orifice. There are frequent occasions in which the VF damper is used along with an girder isolation base.
The tuned mass dampers and the tuned liquid dampers use specified masses or liquids to decrease the sizes of responses under specified modes. In these dampers, since it is possible to increase the sizes of responses under other modes, they are applied to active mass dampers which are a kind of active control system, rather than a passive control system.
Except for the VF dampers, each of which is used along with the girder isolation base, the devices for improving girder performance as described above are limitedly used in bridge structures and have mainly been developed so as to be used for constructional structures (Zahrai et al., 1999).
Meanwhile, recently, efforts have been made to develop a sacrifice member which performs a predetermined structural function to serve as a secondary member while an earthquake does not occur and which passively dissipates energy generated in a structure to improve girder performance when a seismic load is applied.
For example, shear keys and ductile bracings which are installed on ends of a bridge are formed by introducing the concept of the sacrifice member to a seismic load.
The shear keys are devices which function to support horizontal force generated in a direction perpendicular to a bridge axis (a bridge extending direction). The shear keys cause a seismic load to be concentrated in the shear keys which are installed on abutments when an earthquake occurs, and thereby prevent abutments and piers from being damaged. In the shear keys, seismic responses and analyses thereof, and design techniques have been researched through an SSRP (structural systems research project) (Megally et al., 2001).
The shear keys are divided depending upon their shapes into internal shear keys which are installed inside the abutments below a super structure and external shear keys which are installed at sides of the super structure.
In the case of the internal shear keys, while advantages are provided in that it is possible to resist seismic behavior both in the direction of the bridge axis and in the direction perpendicular to the bridge axis, disadvantages are also provided in that it is not easy to gain access to the internal shear keys after installation.
In the case of the external shear keys, while advantages are provided in that it is easy to gain access to the external shear keys, disadvantages are provided in that it is impossible to resist seismic behavior in the direction of the bridge axis.
The devices for improving girder performance of a bridge by using the ductile bracings installed on the ends of the bridge as a sacrifice member are constituted by applying EBFs (eccentrically braced frames), SPSs (shear panel systems), or TADASs (triangular plate added damping and stiffness devices), a kind of ADAS, to the vertical end bracings of steel plate girder bridges. These devices function to dissipate energy generated due to a seismic load applied to a sub structure of the bridge in the direction perpendicular to the bridge axis.
The ductile bracings are designed to be plastically deformed before the sub structure of the bridge reaches a yield point, so that damage due to a seismic load which may be caused in a non-ductile member or a bridge base and bridge seat section can be prevented.
However, these devices are applied on the assumption that the deformation or load generated in the direction of the bridge axis is restrained in some way, and therefore, suffer from defects in that they are incapable of dissipating energy and preventing displacement generated in the direction of the bridge axis due to a seismic load (Zahrai et al., 1999; and Bruneau et al., 2002).
As a result, the conventional girder bridge protection devices as mentioned above encounter problems as described below.
First, it is difficult to apply the conventional girder bridge protection devices to existing bridges and newly constructed bridges, traffic control is necessary to construct the conventional girder bridge protection devices, and it is essential to use costly equipment which is specially fabricated, whereby the economic burden increases.
Second, since the conventional girder bridge protection devices do not normally play a specific role with regard to the behavior of a bridge, if an earthquake does not occur throughout the lifetime of the bridge, the conventional girder bridge protection devices cannot perform any function, whereby economic loss is caused.
Third, it is impossible for the conventional girder bridge protection devices to resist a seismic load in all directions including the direction of the bridge axis and the direction perpendicular to the bridge axis.
Fourth, since it is impossible to precisely predict elastic and plastic behavior of a sacrifice member, it is difficult to secure structural stability.
Fifth, maintenance and repair work cannot be easily implemented for the conventional girder bridge protection devices. Further, when the sacrifice member is damaged, it is not easy to replace the damaged sacrifice member with new one.
In order to cope with these problems, Sang Hyo KIM, the inventor of the present application, has disclosed Korean Patent Laid-open Publication No. 2004-97591 (dated Nov. 18, 2004) entitled “Girder bridge protection apparatus, sacrifice bracing, sacrifice bracing restrainer composing it and reinforcement construction method thereof”.
In the sacrifice bracing described in the published patent document, a central stress concentration section which has a reduced cross-sectional area due to the presence of a notch prevents a shock, generated upon the occurrence of an earthquake, from being transferred to other main parts of the bridge.
In the sacrifice bracing suggested in the published patent document, due to an asymmetrical configuration, it is possible to properly resist vibration which has a level no less than a yield point and basically acts in a direction corresponding to the direction of a bridge axis when an earthquake occurs. Nevertheless, the sacrifice bracing cannot properly resist a seismic shock which acts in the direction perpendicular to the bridge axis.